In the prior art, the applicant of the present application put into practical use a spindle balancer having a gas spring biasing the vertically movable spindle head of a machine tool upward (for example see Patent Document 1). The gas spring of the spindle balancer has a cylinder body, compressed gas introduced in the cylinder body, and a rod inserted in the cylinder body to receive the gas pressure of the compressed gas.
The spindle balancer having a gas spring is generally mounted on a machine tool vertically and, in such a case, the cylinder body is coupled to one of the spindle head and base of the machine tool and the rod is coupled to the other of the spindle head and base of the machine tool.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-105256.